Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (kHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., entitled "Method and Apparatus for Modulating a Light Beam." This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. Typically the deformable elements are supported a distance of .lambda./4 above, and parallel to, the substrate, where .lambda. is the wavelength of the incident light source. When the deformable elements are actuated (for example, when a sufficient switching voltage is applied), they are pulled down and the incident light is diffracted. Optical systems can intercept the diffracted light. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image.
U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, to Bloom et al., entitled "Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate" discloses a method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate. The deformable elements are patterned on top of a sacrificial silicon dioxide film. In order to achieve free-standing beams, the silicon dioxide film is etched away until a supporting frame remains. The last fabrication step provides an aluminum film in order to enhance the reflectance of both the beams and the substrate. To enhance the optical performance of the grating element, dielectric optical coatings may be added using methods that minimize their mechanical effects. The stress can be minimized by alternating materials with compressive and tensile stresses, as discussed by Ennos in "Stresses developed in optical film coatings," published in Applied Optics vol. 5 (1966). Silicon dioxide (SiO.sub.2) evaporated in vacuum at room temperature can be used as a low-index material with compressive stress, and titanium dioxide (TiO.sub.2) evaporated in the same way can be used as a high-index material with tensile stress.
According to the prior art, for operation of the Grating Light Valve (GLV) device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. By modulating the voltage waveform, it is possible to modulate the diffracted optical beam as needed by the specific application. However, a single polarity voltage waveform can lead to device operation difficulties if leakage or injection of charge occurs into the intermediate dielectric layers between the ground plane and the conductor on the ribbons.
The preferred material for the deformable ribbons in the prior art has been silicon nitride. It is a film that can support an intrinsic tensile stress controlled by the deposition process and that is stable, meaning it does not degrade due to stress relaxation or oxidation over time. However one problem encountered with this material is its tendency to trap charge when a bias is applied across it. If this dielectric charge does not dissipate quickly enough after the actuation voltage is turned off, a significant charge accumulation can occur that leads to deterioration in the performance of the device with repetitive actuation.
Charge injection and trapping into insulating dielectric films, such as silicon nitride and silicon dioxide, on semiconductors is well known to occur in various microelectronic devices. Charging effects in silicon dioxide can be minimized by proper deposition, as described, for example, in "Charge transport and transport phenomena in off-stoichiometric silicon dioxide films," J. Appl. Phys. 54, 1983, pp. 5801-5827, by D. J. Marie et al. On the other hand, charge injection into silicon nitride can be used beneficially in non-volatile memories such as the device described by R. T. Bate; see U.S. Pat. No. 4,360,900, "Non-volatile semiconductor memory elements," issued Nov. 23, 1982. Charge trapping phenomena in rolling contact micro-electromechanical actuators have also been reported by C. Cabuz et al. in "High reliability touch-mode electrostatic actuators (TMEA)," Proc. of Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 8-11, 1998, pp. 296-299.
One method to alleviate this problem is to provide an alternating bipolar voltage to the ribbons. A DC-free bipolar waveform produces nearly the same temporal modulation of the diffracted optical beam as the corresponding single polarity waveform while minimizing charge accumulation in the dielectric layers. Stable device operation is thus achieved. However, this complicates the driving circuitry requiring bipolar rather than unipolar driving capability.